Step one would be to redesign the military from the ground-up in a more historical manner. I’ve found that alone solves a fair number of problems.
Step two would have to be decided off Step one, but for now, removing Mortars and restatting Arrow Knights is likely the best approach.
RN:
160 hp
40% ranged resist
3.75 movement speed
32 LoS/30 range
11 ranged damage, x5 artillery
39 siege damage, x2 vs ship/fixed guns, x1.25 walls
To start off, shorten the range. The ridiculous range of the Arrow Knight likely stems from the records of the Aztec tlahhuitolli having a range equal to or larger than the English longbow.
Proposal:
180 hp
30% siege resist
4.25 movement speed
26 LoS/24 range
13 siege damage, x3.5 artillery, 5 DoT/5 sec
25 siege damage, x2 vs ships/fixed guns, x1.25 walls, 5 DoT/5 sec, 2.0 RoF
Additionally, give them a +2 range boost with each upgrade (Champion/Legendary)